Conventional longitudinal magnetic recording is expected to support areal densities of up to around 100 gigabits per square inch before magnetizations in the longitudinal recording media become thermally unstable. Perpendicular magnetic recording media is more thermally stable at higher recording frequencies and is expected to support recording densities of up to around 500 gigabits per square inch. A further increase in areal density, e.g., by a factor of two or three, may be realized by patterning the perpendicular media. However, for densities above that, optical recording will become an attractive alternative to magnetic recording. For instance, effective areal densities exceeding a terabit per square inch should be possible using bitwise volumetric optical data storage. In this context, the effective areal density is the number of bits stored in a volume divided by a surface area on which the volume is projected. Thus, an optical storage medium that stores N layers of recorded data at M bits per square inch per layer has an effective areal density of N*M bits per square inch.
Two-photon recording is an example of a promising bitwise volumetric optical recording technique that may eventually replace conventional magnetic recording. According to this technique, data is recorded by irradiating a selected region of a two-photon recording medium with first and second laser beams. The first and second beams may be intersecting continuous-wave laser beams or high-intensity pulses that are focused in close temporal and spatial proximity to each other in the recording medium. The combined energy of the first and second beams typically changes the molecular or crystalline structure of the irradiated region so that the region will subsequently fluoresce when excited with a readout laser beam of a suitable wavelength. For instance, the recording medium may be fabricated using organic polymers that, in their modified form, are designed to fluoresce in response to specific wavelengths. The emitted fluorescence may be detected for reconstructing the data stored in the recording medium.
The above-noted two-photon writing and readout process is generally described in more detail in the technical paper entitled Multi-layer Optical Data Storage Based on Two-photon Recordable Fluorescence Disk Media, by Zhang et al., published 2001 in the Proceedings of the Eighteenth IEEE Symposium on Mass Storage Systems and Technologies, which paper is hereby incorporated by reference as though fully set forth herein. Moreover, specific organic materials that may be used as volumetric two-photon recording media are described in the publication entitled Materials and Systems for Two Photon 3-D ROM Devices, by Dvornikov et al., published June 1997 in the IEEE Trans-actions on Components, Packaging and Manufacturing Technology, Part A, Vol. 20, No. 2, which publication is hereby incorporated by reference as though fully set forth herein.
FIG. 1A illustrates an exemplary two-photon recording system in which data may be written at a selected region 130 in a volumetric optical recording medium 100 using two intersecting continuous-wave beams. In this exemplary system, first and second collimated beams 110 and 120 are respectively focused by objective lenses 115 and 125. The focused beams intersect at the selected region 130, where their combined energies are absorbed. In accordance with the two-photon writing process, the absorbed energy induces a physical and/or chemical change in the irradiated region 130, which enables the region to fluoresce or not fluoresce in response to readout beams of particular wavelengths. Data can be written at various regions 130 throughout the medium 100 by selectively actuating the lenses 115 and 125 to intersect the first and second beams at different locations in the medium.
FIG. 1B illustrates an energy diagram corresponding to an exemplary two-photon write process 200 and read process 210. For purposes of discussion, assume that the first beam 110 imparts an energy E1 to the irradiated region 130 and the second beam 120 imparts an energy E2. In the write process 200, the energies of the first and second beams are individually insufficient to span an energy gap between a ground state S0 and a stable excited state S2. However the combined energies of the beams (E1+E2) is sufficient to span the gap because of the existence of an intermediate unstable state S1 at energy E1. A photon from the first beam 110 excites an electron from the ground state S0 into the intermediate state S1 and, before the electron can “decay” back to state S0, a second photon from the second beam 120 excites the electron to the stable state S2 where its presence constitutes a structural or molecular change which is the written data.
Thereafter, in the read process 210, a readout beam having an energy E1 may excite electrons from the ground state S0 to the excited state S1. When the excited electrons “relax” back to the ground state S0, they emit a fluorescence (EF) that may be detected. In the read process 210, a region 130 that has been structurally modified by the write process 200 emits less fluorescence than those regions of the optical recording medium that were not effected by the write process. More specifically, the amount of fluorescence emitted from the structurally-modified region 130 is reduced because the population of electrons in the ground state S0 has been depleted by the write process which excited ground-state electrons into the stable state S2.
Because of the difficulties associated with aligning two different beams at a point 130, a single-beam approach is preferred, as shown in FIG. 2A. As shown, a collimated beam 112 is focused by a lens 117 to a point 132 in a two-photon recording medium 102. The energy diagrams for this single-beam two-photon recording process are shown in FIG. 2B. In the write process 202, a photon with energy E1 excites an electron to an unstable intermediate state S1. If the beam intensity is sufficient, a second photon of the beam 112 can ionize the state S1 before the electron decays back to the ground state S0. Specifically, the second photon excites the electron from the intermediate state S1 to a dense band of states S2 from which the electron subsequently relaxes to a stable state S3.
Typically, the irradiated region 132 undergoes a structural or molecular change when its electrons are excited from the ground state S0 to the band of states S2 and then relax to the stable energy state S3. Furthermore, the irradiated region can be modified without significantly altering adjacent data layers in the two-photon recording medium 102. In particular, those skilled in the art will understand that only the material volume at the focal point 132 has a significant number of electrons excited out of the ground state S0 and into the elevated energy state S3, since the two-photon absorption probability is proportional to the beam intensity squared, which in turn is inversely proportional to the fourth power of the distance from the focal plane.
The read process 212 may expose the medium 102 with the same beam 112 used in the write process 202, but with a significantly reduced intensity. The read beam excites electrons from the ground state S0 to the unstable intermediate state S1 with a low probability of further exciting them to the band of states S2. As the electrons decay from the intermediate state back to the ground state, they emit a fluorescence (EF) that is detected by a confocal optics system (not shown). In the region 132 that has been written, many of the ground-state electrons are in the state S3, where they are not subject to the stimulated fluorescence. As such, the written region 132 can be detected by the read beam since its amount of emitted fluorescence is less than the detected fluorescence from the non-written (unmodified) regions in the recording medium 102.
Alternatively, the read process 212 can be performed with a read laser having energy E3 that excites electrons from the ground state S0 to a fluorescing state S4. If the beam energy E3 is less than half of the energy E2 required to excite an electron from the ground state S0 to the band of states S2, the read laser will not be able to unintentionally excite electrons into the stable state S3 which would result in a partial write. However, this readout approach requires a read laser, which differs from the write laser 112, to be incorporated into the same optics path through which the write process 202 is performed.
Although two-photon recording provides the possibility of storing thousands of data layers in a volumetric optical recording medium that corresponds to effective areal densities of up to 100 terabits per square inch, two-photon recording is currently limited by spherical aberrations that occur during the writing and reading processes. FIGS. 3A-B illustrate the undesired effects of spherical aberration in a refractive material 330 that may be used as a two-photon recording medium. FIG. 3A illustrates a collimated laser beam 300 that is focused by an objective lens 310 to a spot 320. FIG. 3B illustrates the same optical configuration, except the lens 310 converges the beam 300 to a spot 340 within the refractive material 330. In this case, spherical aberration in the refractive material 330 causes the focused spot 340 to “spread” to a larger diameter than it would otherwise exhibit in the absence of the material 330. Thus, the spherical aberration inherently limits the ability to focus the incident beam 300 within the refractive material.
Referring again to FIG. 1A, spherical aberration in the two-photon recording medium 100 may prevent the first and second beams 110 and 120 from being sharply focused at the selected region 130. As such, the first and second beams may not impart a sufficient amount of energy (E1+E2) to induce the intended structural or molecular change in the irradiated region 130. Similarly, spherical aberration in the single-beam system of FIG. 2A also may limit the ability of the write beam 112 to excite electrons to the stable state S3 via the band of states S2. Additionally, the spherical aberration also may prevent a readout laser from being focused sharply enough to impart a sufficient amount of energy to induce fluorescence during the readout processes 210 and 212.
The above-noted writing and readout limitations caused by spherical aberration are compounded by the inherent difficulty of removing the aberration at different depths in an optical recording medium. As is known in the art, the amount of spherical aberration is typically depth dependent, such that even when correction occurs for aberration at one focal depth, the correction does not prevent spherical aberration in various degrees at other focal depths. The depth dependence of spherical aberration therefore limits the volume in which data can be effectively written or read back without active changes in the system to essentially offset the aberration. Typically, the useful depth range in a conventional two-photon recording medium is only about 1 millimeter. That is, data can be effectively stored in data layers situated within about a millimeter of a focal plane before spherical aberration becomes prohibitive to the write and read processes even with use of known techniques for correcting spherical aberrations.
There are many well-known techniques that correct for spherical aberration when focusing at different physical depths in a volumetric optical data storage medium, such as a two-photon recording medium. The techniques generally involve actively repositioning or adjusting system components that are dedicated to the correction of the spherical aberration. Accordingly, the system requires separate actuations for focusing and for removing spherical aberration.
For example, consider U.S. Pat. No. 5,202,875 to Rosen. As is typical, the Rosen recording system includes a focus servo that actuates an objective lens in the optical path so as to minimize a detected focus error signal. By moving the objective lens in this fashion, light may be focused at a plurality of discrete information layers in a storage medium. To correct for spherical aberration introduced by the act of focusing, Rosen separately actuates a “multiple data surface aberration compensator” that actively adjusts by discrete levels the amount of refractive material added to the optical path. In this manner, the compensator can be used to select among several thicknesses of refractive material for insertion in the optical path in order to offset spherical aberration introduced when changing focal planes from one recording layer to another.
Other known aberration correction techniques insert stationary optical elements into the optical path. For instance, U.S. Pat. No. 6,795,248 to Kimura teaches insertion of a stationary aberration-compensating element having a diffractive structure that includes concentric gratings designed to offset spherical aberration before incident light is focused in an optical recording medium. Solutions that utilize stationary optical elements, such as Kimura's diffractive structure, typically do not remove the spherical aberration over a range of focal depths, but merely attempt to reduce it to an acceptable level at various depths. Indeed, the stationary aberration-correcting elements are often designed primarily to correct for chromatic aberration, and correction for spherical aberration is a secondary concern.
It is therefore generally desirable to provide an improved technique for correcting for spherical aberration in a volumetric optical data storage medium, such as a two-photon recording medium. The technique should avoid separate actuations for focusing and correcting for spherical aberration. Further, the technique should reduce spherical aberration more effectively at various depths than is possible using conventional stationary compensation elements. The technique should not only correct for spherical aberration at discrete recording layers, but also over a continuous spectrum of depths.